A sensitive and specific radioimmuneassay which will allow measurement of physiological levels (femtomolar quantities) of leu5-beta-endorphin and met5-beta-endorphin will be developed and tested for cross reactivity with a number of related and unrelated brain and serum peptides. Chemical methods, including antibody purification and CNBr-modification of tissue samples, will be used to increase the sensitivity and specificity of the test. Both clinical and basic physiological inquiries will be facilitated by the availability of this assay.